1. Field of the Invention
The present invention relates to a system and method for cancelling harmonics of a digital system clock frequencies when the harmonics are coupled with an incoming signal.
2. Description of the Related Art
In any digital integrated circuit or system there is typically a running digital clock. Non-linear effects in the system typically cause the base digital clock frequency to lead to harmonics of that clock frequency. For example, if the base clock sequence is 100 MHz, some processes on the chip may cause undesirable components of 10 to 20 harmonics that can occur at 1 MHz and 2 MHz respectively and 1 GHz and 2 GHz respectively from the 100 MHz clock. These undesirable harmonics are also known as spurs. In a particular system, one can accurately predict the exact frequencies of the spurs because of the known base clock and harmonic frequency.
In a fast band system, i.e., a system that is operating at a high frequency with a fast band signal around the center carrier frequency, the fast band signal is down converted to base band by a radio chip. A base band processor then processes the base band signal. Specifically, the base band signal is digitized and a digital signal processor processes the base band signal. The digital signal processor usually works the running digital clocks. Therefore, when the RF section and the digital section are put on the same chip, the incoming digitized signal and the clock signal can couple easily. As such, the digital clock may create spurs all the way up to the fast band systems region of operation. Therefore, at the input of an RF, a desired signal or packet will be coupled with the spur frequency. The desired signal is typically a spread out signal, for example, 20 MHz in bandwidth. The spur frequency, on the other hand, is a very narrow band which is determined by the harmonic of the clock frequency.
A typical method of suppressing harmonics or spurs is through the use of electrical isolation between the digital system clock and the RF circuitry. However, isolation is not completely successful, and the in-band spurs can create numerous problems including carrier detection problems such as false detection. Additionally, the spurs created by the harmonics can degrade the signal-to-noise ratio. As the signal-to-noise ratio decreases, there is usually a higher incidence of dropped packets.